Why does my water pressure go up when the hot water heats?

What is happening

Water expands as it heats. Taking a storage tank from cold to around 60 degrees grows the water by roughly two per cent, so a 250 litre tank ends up with four or five litres more than it has room for. Water barely compresses, so in a sealed system those litres push the pressure up fast.

A hot water system is built for this. The valve set on its cold inlet includes a non-return valve, which lets water in but not back out, so the expansion stays on the hot side. When the pressure there reaches the relief setting, a relief valve lets the extra water out to a drain. That is why a hot water drain often dribbles while the tank heats. The cold pipes never feel it.

Why it spreads to the whole house

If the non-return valve on the hot water inlet is worn or missing, the expanding water has another way out: backwards into the cold pipes. On mains water that usually goes unnoticed, because the street main is big enough to soak it up.

On a tank and pump system it cannot go back that way. A pressure pump has its own non-return valve, so everything after the pump is sealed, and the expansion raises the pressure in every pipe in the house, hot and cold, until the water cools or something gives.

The same thing can happen through a crossover, anywhere hot and cold meet. A tempering valve, which blends bathroom hot water down to 50 degrees, has small non-return valves inside that wear out. Mixer tap cartridges and thermostatic shower mixers can do it too. Everything still works normally, which is why it goes unnoticed for years.

How to tell it is the hot water

Timing is the giveaway. Electric hot water on an off-peak or controlled-load tariff heats in set windows, usually overnight, and in many areas again in the middle of the day when rooftop solar is cheap. Solar and timer-controlled heat pump systems have their own patterns. If the pressure climbs at the same times each day, settles at a similar peak, then eases off slowly as the water cools, that is heating, not a leak and not the pump.

One check rules out the obvious: the rise happens while the pump is resting. A pump can only push pressure up while it runs, and it stops at its set pressure, so a climb well past that with the motor off is coming from somewhere else.

Is it doing any damage?

Not quickly. Taps, toilet valves and flexible hoses are built to cope with more than this, and plenty of older homes on high mains pressure sit at similar levels all day. It is still worth fixing. Plumbing rules generally cap the pressure at outlets inside a home at 500 kPa, and pressure that climbs and falls every day wears tap washers, toilet cistern valves, braided hoses under sinks and appliance inlet valves faster than a steady supply would.

The exception is a tempering valve. If hot water is crossing over through one, its internal valves have failed, and it may not be limiting bathroom hot water to a safe temperature. If you have young children or older people at home, get that checked sooner.

Your pump is not harmed. It waits in standby until the pressure drops below its cut-in, then carries on as normal.

What a plumber will check

1. Fit a pressure gauge to a cold tap, such as the washing machine tap, during a heating window, and confirm the rise.
2. Close the isolation valve on the hot water system’s cold inlet. If the cold side stops rising, the inlet valve is the culprit, and it is an inexpensive part to replace.
3. If it keeps rising, look for a crossover, starting with the tempering valve and then mixer taps.
4. Check the relief valve on the hot water system opens and drains as it should.
5. Where it suits the system, fit a small pressure vessel near the pump to absorb the expansion, which also means the pump starts less often.

How a Grewa Boost spots it

A Grewa Boost reports its pressure all day, so it sees a climb that nobody in the house would notice. When the pressure rises well past the set point while the pump is resting, on several days in a week, the app shows a calm notice that it usually points to the hot water system, with what to ask your plumber. It only counts pressure that climbs while the pump rests, so turning your own set pressure down does not set it off. If the pressure ever reaches 800 kPa, the notice turns red, and we are told too.

Common questions

Does this happen on mains water too?

The expansion happens on every storage hot water system. On a correctly installed one, the non-return valve keeps it on the hot side and the relief valve lets it out. If that valve is missing on mains, the expansion usually pushes back into the street main and nobody notices. It shows up on tank and pump systems because the pump’s own non-return valve seals the house.

Is a dripping hot water relief drain a problem?

A small dribble while the tank heats is that valve doing its job. A constant flow, or water running when the system is not heating, is worth a plumber’s look.

Could it be my pump?

Not if the climb happens while the pump is resting. A pump only adds pressure while it runs, and it stops at its set pressure. If the rise also lines up with your hot water’s heating times, the pump is ruled out.

Could I have turned the pressure up by mistake?

A higher set pressure makes the pump run to reach it, and the pressure then sits steady at the new level. It does not climb on its own while the pump is off, so that pattern points elsewhere.

Will a pressure limiting valve fix it?

Not on its own. A pressure limiting valve controls what comes into the house. This pressure is made inside the house by the heating water, so the fix is at the hot water system: a working inlet valve, a working relief valve, and if needed somewhere for the expansion to go.